1. Field of the Invention
This invention relates to methods of treating permeable subterranean formations in oil and gas wells using polymer gels to reduce the formation permeability, and more particularly to a method of treating such formations with such polymer gels to selectively reduce the permeability in certain regions while maintaining the permeability in others.
2. Description of the Prior Art
In oil and gas wells, it is sometimes desirable to block or reduce the permeability of certain zones or regions of the subterranean formation surrounding the well. For example, water from water-bearing zones can interfere with oil or gas recovery operations. The production of water from these zones reduces the amount of hydrocarbons that can be recovered from the well and may require additional separation and disposal procedures if the water is removed from the well along with the oil and gas. Thus, it is beneficial to block off these water-bearing regions so that the desired oil or gas can be recovered more effectively. In other instances, such as in injection wells, it may be desirable to block off certain zones to plug high-fluid-loss zones or layers, change fluid injection profiles and the like.
Polymer gels formed from hydrophilic polymers, such as polyacrylamides, have been used in the past to reduce the permeability of selected zones within subterranean formations in oil and gas wells as disclosed in U.S. Pat. No. 5,338,465, issued Aug. 16, 1994 to Thomas P. Lockhart. This patent is hereby incorporated in its entirety by reference. These polymers are usually mixed with water at the surface to produce aqueous solutions of gellable polymers that are pumped into the formation. A cross-linking agent is typically added to the solution at the surface, along with any delaying agents or gel inhibitors, so that once the solution is in place within the desired region of the formation, the polymer begins to gel or cross-link so that the polymer gels in place and plugs the pore channels and flow paths of the formation and the permeability of the formation is effectively reduced or blocked. In this way, the water-bearing zones or high-fluid-loss zones and the like, as discussed previously, can be plugged or blocked.
One of the problems encountered in treating formations with these polymer gels is that the treatments are relatively non-selective and can result in blocking or plugging zones, such as hydrocarbon-bearing zones, where it is desirable to maintain permeability. In many cases, the production of all fluids from the formation is stopped. One method of repairing such damaged areas is to re-perforate the damaged zone (typically the hydrocarbon-bearing zone or layer). If the gel penetration is too deep to perforate past the gel damage, the well may have to be abandoned, sidetracked to bypass the damaged zone or the like. Such procedures are both time consuming and costly.
Another method of restoring the damaged area is to inject gel-breaker materials into the gel-filled zones to break the gel. These methods are substantially less costly. The gel breaker can usually be placed within the damaged zone through coiled tubing, through the production tubing of the well or the like. The prior art gel breaker systems are water-based and usually consist of a chelating agent dissolved in an aqueous solution. Because such gel breakers are water-based, they have an affinity for the water-bearing zones of the formation. The injected gel breaker thus has a tendency to penetrate the water-bearing zones, breaking down the gel within these zones so that there is an increase in water production which may exceed the desired increase in hydrocarbon production. The quantity of the aqueous gel-breaking solution necessary to ensure that the gel within the hydrocarbon-bearing zones is effectively broken down may result in an undesirable removal of the gel from the water-bearing formations with a resulting restoration of a substantial portion of the initial water production.
What is therefore needed is an effective and reliable means of treating a subterranean formation in an oil or gas well with polymer gels to plug or block water-producing zones, while hydrocarbon-bearing zones of the formation remain open so that oil and gas can be recovered more effectively, or a method for selectively removing gels from hydrocarbon-bearing formations.